He's one of the funniest - and most famous - men on the planet. But his Hollywood stardom has come at a price.

He's one of the funniest - and most famous - men on the planet. But his Hollywood stardom has come at a price.

Famously sober for 20 years after battling drink and cocaine for most his adult life, Robin Williams has revealed how he succumbed to his demons again - and destroyed his marriage.

"I was filming in Alaska," he says.

"I was away from everything, I was kind of isolated from things I normally get to do, like ride my bike, seeing friends and family.

"All of a sudden I thought, 'I've been 20 years sober, I can drink a little'. It's like, 'I can be somewhat circumcised'.

"It's that idea that you can have one drink - and no you can't. Within a week I was drinking heavily. It was so quick that even I was like, 'Wow'.

"Because you have that initial warm feeling going, 'Oh, I remember this'. And your body does, too. And your body goes, 'Yeah, so do I'.

"Then the demon voice comes, 'Yeah, so do I. You know what would be great? You know we bought a little bottle before? A full bottle would be wonderful'."

Williams tried - but failed - to mask his drinking. When a bartender questioned him, he pretended his drink was for a friend.

"I went behind a pole and went..." He knocks back an invisible shot. "I then went back for another one for my 'friend'.

"I thought I was fooling people. But it's the old thing of 'they say vodka doesn't smell'. No, not until you sweat. And you just lie and lie and you think 'I can deal with this'. And then you finally go, 'No you can't'. And then you give up."

His concerned family forced him to take action and enter rehab four years ago. And his 27-year-old son Zak believes Williams would now be dead if he had carried on drinking.

"Thank God they acted," says Williams, 59. "I was living off the lie that I could do it alone. Eventually you go, 'You can't'. You're sober but you're not dealing with issues like, 'What makes you do this? Why

And that's when you have to really admit, 'I'm an alcoholic. I can't drink'.

"So you go and you surrender, and you go to a starting place, almost like boot camp, total lockdown. And you go through that and you come out going, 'OK, there are others with this thing and I can work with this, and that will help'." Williams shot to fame in the late Seventies as a kooky alien in the sitcom Mork & Mindy, before becoming known as a stand-up comic.

His astonishing performance in the film Good Morning, Vietnam earned him an Oscar nomination in 1988, with two more in the following five years, for Dead Poets Society and The Fisher King.

Mrs Doubtfire - in which he dressed as a woman - brought mainstream success. And in 1998 he finally won an Oscar, for his role in Good Will Hunting.

But along with fame came addiction.

One of his lowest points had been a New Year's Eve party at Woody Allen's house in the Seventies. Robin says: "I ended up talking to this big fat guy and it turned out to be Robert De Niro, who was getting ready for Raging Bull. I said, 'Hey, who's the big fat f*****?' "Woody replied, 'That was Robert De Niro. You know, you have to go now. You just urinated'."

After realising he was in big trouble, Robin finally managed to wean himself off drink and drugs by himself, going cold turkey.

Two things had forced him to take control of his behaviour. The first was the death of his good friend, the Blues Brothers actorcomedian John Belushi, from a lethal injection of heroin and cocaine in 1982. Williams had been one of the last people to see him alive. He says: "John was like a bull and that he died was like, 'Oh, dude'. That scared me." The actual turning point came, though, when Robin's then wife, Valerie Velardi, fell pregnant with his first child, Zak.

He knew then that he had to clean up his act. "I went, 'I can't be coked off my t**s going, 'Daddy loves you. Want to play with Daddy? Let's play with Daddy'. That was why I sobered up the first time - I had a biological imperative."

Williams admits with admirable understatement that he is tricky to live with. He and Valerie divorced in 1988 after 10 years together.

The following year he married Marsha Garces, Zak's nanny, who became his personal assistant on the set of Good Morning, Vietnam.

They have both always insisted they were no more than friends until his 1988 divorce.

But when they married a year later, Marsha was already pregnant. They went on to have two children of their own - Zelda, 21, and Cody, 18.

Sadly, when he fell back into addiction after 20 years sober, Williams managed to beat his alcohol addiction - but he couldn't save his second marriage. Marsha filed for divorce in March 2008, after 19 years of marriage, citing irreconcilable differences. Williams, who still attends weekly Alcoholics Anonymous sessions, admits: "You know, I was shameful, and you do stuff that causes disgust, and that's hard to recover from. "You can say, 'I forgive you', and all that stuff, but it's not the same as recovering from it. It's not coming back."

Alone, and with no film scripts coming in, Williams went on tour with a new stand-up show, Weapons of Self Destruction, in which he drew on his recent experience of addiction.

But 30 dates into the run he started experiencing breathing difficulties. An angiogram revealed that he needed heart surgery.

After being successfully fitted with a cow's heart valve, he took to the road again last September for a further 50 dates.

Williams saw his operation as a second chance - a new beginning.

"It was kind of helpful to deal with all the s**t that had gone on in my life, to talk about that. And then after the heart surgery it became even more kind of, 'Yeah, baby. You're alive! You're alive! Talk about that, motherf*****!'" But while his stage show was well received, his first film after rehab, Old Dogs, was largely dismissed by critics.

But his follow-up, World's Greatest Dad, written and directed by his friend of 30 years Bobcat Goldthwait (who played Zed in the Police Academy series), has earned him some of his best reviews in years.

Shot on a shoestring budget, the film is a pitch-black comedy in which Robin plays a high school teacher who uses a family tragedy to further his own dreams of literary success.

It is a far cry from his family-oriented Mrs Doubtfire-esque comedies. But it was perhaps what he needed.